Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
Metal oxides with the general formula AxMO2 (where A represents one or more alkali metal ions and M represents one or more metal ions at least one of which has several oxidation states, for example a transition metal) are known to crystallise in a number of different layered structures. This is described in detail by C. Delmas et al in “Structural Classification and Properties of the Layered Oxides”, Physica 99B (1980) 81-85. In summary, the structures are all made up of MO6 edge sharing octahedra which form (MO2)n sheets. These sheets are stacked one on top the other and are separated by the alkali metal atoms and the exact position of the alkali metal will dictate whether the overall structure of the metal oxide is to be described as octahedral (O), tetrahedral (T) or prismatic (P). In a lattice made up of hexagonal sheets, there are three possible positions for the oxygen atoms, conventionally named A, B and C. It is the order in which these sheets are packed together that leads to the O, T and P environments. The number 2 or 3 is also used to describe the number of alkali metal layers in the repeat unit perpendicular to the layering. For example, when the layers are packed in the order ABCABC, an 03 structure is obtained. This translates to 3 alkali metal layers in the repeat unit and each alkali metal being in an octahedral environment. Such materials are characterised by the alkali metal ions being in octahedral orientation and typical compounds of this structure are AxMO2 (x≤1). The order ABAB with the alkali metal ions in tetrahedral orientation will yield a T1 structure which is typified by A2MO2 compounds. Packing the sheets in ABBA order gives a P2 structure in which one half of the prism shares edges with MO6 octahedra and the other half shares faces and typical compounds are A=0.7MO2. And finally, packing in ABBCCA order results in a P3 structure type in which all prisms share one face with one MO6 octahedron and three edges with three MO6 octahedra of the next sheet. A=0.5MO2 compounds are found to adopt the P3 structure. It will be noted that the amount of alkali metal present in the AxMO2 material has a direct bearing on the overall structure of the metal oxide.
Over the last ten years, numerous workers have investigated the electrochemical properties of single phase metal oxides with a P2 structure. For example C. Delmas et al report the phase transformations and electrochemical behaviour of P2-NaxCoO2, see for example J. Solid State Chem., 2004, 177, 2790-2802 and Inorg. Chem., 2009, 48, 9671-9683. Further, Lu and Dahn, J. Electrochem. Soc., 2001, 148, A710-715, demonstrate that the P2-layered oxide Na2/3[Ni1/3Mn2/3]O2 can reversibly exchange Na-ions in sodium half cells however, these oxide compounds are expected to show poor cycling ability, especially between 2.3-4.5 V at C/100.
More recently, Kim et al Adv. Energy Mater., 2011, 1, 333-336 reported that the presence of lithium in single phase P2 lithium substituted compounds such as Na1.0Li0.2Ni0.25Mn0.75O2, provides some improvement in the structural stability during cycling, but the reversible capacity of these compounds is still too low due to the limited amount (25%) of redox active divalent Ni. And in another recent paper by Y. Shirley Meng and D. H. Lee, Phys. Chem. Chem. Phys., 2013, 15, 3304, P2-Na2/3[Ni1/3Mn2/3]O2 is reported to exhibit excellent cycling and a high rate capability, however these results are only achieved when the material is charged below 4.22V; above 4.22V, the charge capacity is not maintained during cycling due to the phase transformation from P2 to O2.
In conclusion, the metal oxides that are discussed above are hampered by poor cycling stability, especially across a wide range of charge voltages. As a consequence, the commercial application of these compounds in Na-ion cells is limited.
The current workers have developed novel compounds which are doped-nickelate-containing materials (hereafter referred to as Target Active Materials) that are capable of delivering specific capacity performance with little or no fading on cycling. Moreover ideally, the doped nickelate-containing materials of the present invention have been found to achieve these excellent results under voltage conditions that would typically result in the phase transformation from P2 to O2; this is a significant improvement over compounds described in the prior art. Thus the Target Active Materials of the present invention may be used to provide an electrode, preferably a positive electrode, which is able to be recharged multiple times without significant loss in charge capacity. In particular the Target Active Materials of the present invention will provide an energy storage device for example a battery for use in a sodium-ion cell or a sodium metal cell.
The present invention therefore provides doped nickelate-containing materials (Target Active Materials) with the general formula:AaM1vM2wM3xM4yM5zO2-δ                wherein        A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        wherein        0.4≤a<0.9, preferably 0.5≤a<0.85, further preferably 0.6≤a≤0.75,        0<v<0.5, preferably 0<v≤0.45 and ideally 0<v≤0.333,        at least one of w and y is >0,        x>0,        z≥0,        0≤δ≤0.1,        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality.        Preferably δ=0.        
Preferred doped nickelate-containing materials (Target Active Materials) are of the general formula:AaM1vM2wM3xM4yM5zO2-δ                wherein        A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        wherein        0.55<a<0.85,        0.25<v≤0.333,        at least one of w and y is >0,        x>0,        z≥0,        0≤δ≤0.1,        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality.        Preferably the alkali metal A is selected from either sodium or a mixed alkali metal in which sodium is the major constituent.        
In particularly preferred Target Active Materials, v+w+x+y+z=1.
Preferred Target Active Materials include:                Na0.67Ni0.3Mn0.6Mg0.033Ti0.067O2,        Na0.67Ni0.267Mn0.533Mg0.0067Ti0.133O2,        Na0.67Ni0.283Mn0.567Mg0.05Ti0.1O2,        Na0.67Ni0.25Mn0.667Mg0.083O2,        Na0.7Ni0.240Mn0.533Mg0.110Ti0.117O2         Na0.6Ni0.240Mn0.533Mg0.060Ti0.167O2         Na0.67Ni0.240Mn0.533Mg0.093Ti0.133O2         Na0.55Ni0.240Mn0.533Mg0.035Ti0.192O2         Na0.67Ni0.240Mn0.533Mg0.043Ti0.083Fe0.100O2 and        Na0.67Ni0.240Mn0.533Mg0.043Ti0.083Al0.100O2.        
Metals M2 and M4 may be the same or different metal(s) in oxidation state 4+. Moreover M2 and M4 are interchangeable with each other. When M2=M4, then the structure may be written either as:AaM1VM2WM3XM4YM5ZO2-δ,orAaM1VM2W+YM3XM5ZO2-δ,orAaM1VM3XM4Y+WM5ZO2-δ,and all of these forms of the equation are to be regarded as equivalent.
Preferably the doped nickelate-containing materials of the present invention (Target Active Materials) comprise sodium alone as the chosen alkali metal.
Also, in further preferred doped nickelate-containing Target Active Materials, M2 comprises one or more metals in oxidation state 4+ selected from manganese, titanium and zirconium; M3 comprises one or more metals in oxidation state 2+ selected from magnesium, calcium, copper, zinc and cobalt; M4 comprises one or more metals in oxidation state 4+ selected from manganese, titanium and zirconium; and M5 comprises one or more metals in oxidation state 3+ selected from aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.
Target Active Materials comprising a layered P2-type structure are particularly advantageous.
The Target Active Materials may be prepared by any known and/or convenient process. For example, one or more precursor materials for the Target Active Materials may be heated (for example in a furnace) in order to facilitate a solid state reaction process. Such a process may be conveniently performed in the presence of air, but it may also be performed under an inert atmosphere. Ideally, the one or more precursor materials for the Target Active Materials comprise one or more metals selected from A, M1, M2, M3, M4 and M5, which are as defined above. Particularly preferably, these one or more metals are present in the precursor materials in a stoichiometric ratio that corresponds with the amounts of the respective one or more metals present in the Target Active Material.
The doped nickelate-containing materials (Target Active Materials) of the present invention are suitable for use in many different applications including sodium ion and/or lithium ion and/or potassium ion cells which may be widely used for example in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices.
Advantageously, one or more Target Active Materials may be used in an electrode, preferably a positive electrode (cathode), and further preferably in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).
It is also possible to convert sodium-ion derivatives into mixed lithium-ion/sodium-ion materials using an ion exchange process.
Typical ways to achieve Na to Li-ion exchange include:
1. Mixing the sodium-ion material with a lithium-containing material e.g. LiNO3, heating to above the melting point of LiNO3 (264° C.), cooling and then washing to remove the excess LiNO3 and side-reaction product
2. Treating the Na-ion material with an aqueous solution of lithium salts, for example 1M LiCl in water; and
3. Treating the Na-ion material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.